Boundary layer energiser

ABSTRACT

A boundary layer energiser ( 20 ) for energising a boundary layer flow ( 33 ) over a surface ( 22 ), the boundary layer energiser ( 20 ) comprising one or more passages ( 24 ) terminating in one or more respective holes ( 26 ) provided on the surface ( 22 ), wherein the one or more passages ( 24 ) comprises at least one fin ( 25 ) configured so as to generate a vortex flow in the one or more passages ( 24 ) such that, when in use, a fluid emanating from the one or more passages ( 24 ) flows in a vortex.

This invention relates to a boundary layer energiser for energising aboundary layer flow over a surface.

BACKGROUND

Typically, active flow control involves injecting an energetic flow intoa boundary layer to increase the momentum of the boundary layer flowthereby delaying separation of the boundary layer from the surface. Inparticular, active flow control on the intake lips of a jet engine hasbeen proposed previously.

It has been suggested previously that active flow control could beprovided by the use of perforations on the intake surface. A flow may beintroduced through these perforations to re-energise the intake nearwall boundary layer. These perforations may take the form of holeseither perpendicular to the surface contour or angled in a uniformorientation. However, as shown in FIG. 1 such perforations produceplumes 10 which project into the mainstream flow 12 and generate arelatively weak horseshoe vortex 14 at the base, which is used tore-energise the boundary layer downstream. With this arrangement a highflow of air is required in order to re-energise the boundary layersufficiently and this reduces the efficiency and performance of theengine. The source of this problem is that it takes a lot ofpower/energy to create the plumes, which in turn create relatively lowpowered horseshoe vortices that re-energise the boundary layer. Thus thepreviously-proposed mechanism generates high-powered jets and lowpowered vortices, but it is the vortices that do the useful work.

By contrast, U.S. Pat. No. 4,749,150 discloses a “Turbofan duct withnoise suppression and boundary layer control” and describes the use ofsuction through the acoustic liner of an engine intake for boundarylayer control. However, this invention also uses a lot of power to sucksufficient air from the boundary layer. Furthermore, it is also subjectto blockage with debris such as dust and insects. For these reasonsblowing, as opposed to suction, is preferred.

The present invention therefore seeks to address these issues.

STATEMENTS OF INVENTION

According to a first aspect of the present invention there is provided aboundary layer energiser for energising a boundary layer flow over asurface, the boundary layer energiser comprising one or more passagesterminating in one or more respective holes provided on the surface,wherein the one or more passages are configured such that, when in use,a fluid emanating from the one or more passages flows in a vortex.

The one or more passages may comprise an inwardly facing fin arranged ina spiral. The fin may be configured so as to generate a vortex flow inthe one or more passages. The boundary layer energiser may comprise aplurality of passages. Each passage may terminate in a respective holeprovided on the surface. The holes may be arranged in a cluster on thesurface. The plurality of passages may be angled with respect to oneanother at the surface such that, when in use, the vortex may be formedby the fluid flowing through the plurality of passages.

The flow may be provided by a flow source. The flow may be provided froman opening in a second surface.

A turbomachine may comprise a boundary layer energiser as describedabove. A gas turbine may comprise a boundary layer energiser asdescribed above.

According to a second aspect of the present invention there is provideda method of energising a boundary layer flowing over a surface, themethod comprising: providing one or more passages terminating in one ormore respective holes provided on the surface; allowing a fluid to flowthrough the plurality of passages; and forming a vortex with the fluidby virtue of the one or more passages which are configured such that thefluid emanating from the one or more passages flows in a vortex.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings, in which:

FIG. 1 shows a previously proposed boundary layer re-energiser;

FIG. 2 shows a boundary layer energiser according to an example of thepresent invention;

FIG. 3 shows a plurality of boundary layer energisers according to anexample of the present invention in a first arrangement;

FIGS. 4 a and 4 b shows a plurality of boundary layer energisersaccording to an example of the present invention in a secondarrangement;

FIG. 5 shows further detail of the boundary layer energisers in thesecond arrangement;

FIG. 6 shows a view of a boundary layer flow looking in a stream-wisedirection and downstream of a boundary layer energiser according to anexample of the present invention;

FIG. 7 shows a first example application for a boundary layer energiseraccording to an example of the present invention;

FIG. 8 shows a second example application for a boundary layer energiseraccording to an example of the present invention;

FIG. 9 shows a third example application for a boundary layer energiseraccording to an example of the present invention;

FIG. 10 shows a fourth example application for a boundary layerenergiser according to an example of the present invention;

FIG. 11 shows further detail of the fourth example application for aboundary layer energiser according to an example of the presentinvention;

FIG. 12 shows a fifth example application for a boundary layer energiseraccording to an example of the present invention;

FIGS. 13 a and 13 b shows a sixth example application for a boundarylayer energiser according to an example of the present invention;

FIG. 14 shows a seventh example application for a boundary layerenergiser according to an example of the present invention; and

FIGS. 15 a and 15 b shows further detail of the seventh exampleapplication for a boundary layer energiser according to an example ofthe present invention.

DETAILED DESCRIPTION

With reference to FIG. 2, a boundary layer energiser 20 according to anembodiment of the present invention, energises a boundary layer flowingover a surface 22. The boundary layer energiser comprises one or morepassages 24 and each passage 24 terminates in a respective hole 26provided on the surface 22. The one or more passages 24 are configuredsuch that, when in use, a fluid emanating from the one or more passagesflows in a vortex. In one embodiment, and as shown in FIG. 2, the one ormore passages 24 comprises an inwardly facing fin 25, the fin 25 beingconfigured so as to generate a helical vortex flow in the one or morepassages 24, either in a clockwise or anticlockwise direction. The fin25 may be arranged in a spiral. The flow provided to each of thepassages 24 is provided by a flow source (not shown).

The vortex generated by the passages 24 may, in the absence of amainstream flow, either be perpendicular to the surface or have acomponent parallel to the surface.

With reference to FIGS. 3, 4 a and 4 b, the boundary layer energiser 20may comprise a plurality of passages 24, which may, for example, bearranged in a linear pattern (FIG. 3) or a hexagonal pattern (FIGS. 4 aand 4 b). In either case it is desirable that neighbouring passages 24generate vortices that rotate in opposite directions. As is shown inFIG. 4, a plurality of hexagonal patterns may be provided, and these mayin turn be arranged in a line. The linear arrangement of passages 24 orthe linear arrangement of hexagonal patterns of passages 24 may bedisposed so that the linear arrangement is substantially perpendicularto the direction of the mainstream flow 30.

With the linear arrangement of individual passages 24 (as shown in FIG.3), a gap 31 is preferably provided between neighbouring passages 24where the tendency of the counter rotating neighbouring passages 24 isto generate a flow opposing the mainstream flow. The gap allows themainstream flow to flow between these neighbouring passages 24. Such agap is not however required between neighbouring passages 24 where thetendency of the counter rotating passages 24 is to generate a flow inthe direction of the mainstream flow.

With reference to FIG. 5, the boundary layer energiser 20 may bearranged in patterns to produce meshed geared vortices. (FIG. 5corresponds to one of the hexagonal patterns shown in FIG. 4). A space32 is provided in the centre of the pattern of passages 24 to allow themainstream flow 30 through. Any shape with passages 24 disposed at thevertices may be used, however an even number of vertices would ensureneighbouring passages 24 generate vortices that rotate in oppositedirections. With such an arrangement the vortices reinforce rather thanact against each other. This allows the vortices to persist in themainstream flow rather than dying out. The vortices therefore require alower energy flow to drive them. Thus, for a given driving pressure, theboundary layer energisers of the present invention generate stronger andmore persistent vortices than conventional holes, which generate thehorseshoe type vortices described above.

With reference to FIG. 6, the boundary layer flow 33 near the surfacewall 22 contains low energy air which tends to separate easily from thesurface causing a flow breakdown. In the specific example of a jetengine intake, such a flow separation prevents the intake from workingeffectively. The boundary layer energiser 20 acts to re-energise theboundary layer and assists in driving the boundary layer along thesurface, thereby preventing it from separating. The boundary layerenergisers 20 start out with the axes of the vortices substantiallynormal to the surface 22, but the vortices are soon turned byinteraction with the mainstream flow 30 to a substantially axialdirection (tangential to the surface). The powerful helical vorticesproduced by multiple passages 24 introduce powerful stirring of the nearwall boundary layer 34 allowing fluid(eg air) to be drawn from themainstream flow towards the surface and vice versa, thereby reenergisingthe boundary layer with the mainstream flow.

With reference to FIG. 7, the flow (for example air) being ejected fromthe boundary layer energiser 20 may be hot (ie for anti-icing purposes).The stirring mechanism described above will promote powerful mixing toensure rapid temperature decay of the hot flow emanating from the holes.This rapid temperature decay prevents thermal damage to downstreamstructures 40 that may be made from low temperature capability materialssuch as aluminium or carbon reinforced plastic.

With reference to FIG. 8, one or more boundary layer energisers 20 maybe arranged on an intake 50 of a jet engine 52. The boundary layerenergisers may be disposed, at least in part, about the circumference ofthe intake cowling. In particular, the boundary layer energisers may bedisposed on the intake lower lip 54 to reduce incidence separation andthe boundary layer energisers may be disposed on the intake lip sides 56to reduce crosswind separation. Such an arrangement improves intake lipflow stability by preventing intake flow separation thereby protectingthe fan from damage. This is particularly important in conditions ofcross-wind on the ground and high incidence in flight. This is achievedwith lower active flow rates or using lower energy flow supply than theprior art. Applying the boundary layer energisers to the jet engineintake 50 allows a smaller and more efficient intake to be designed thusreducing weight and improving fuel consumption.

With reference to FIG. 9, one or more boundary layer energisers 20 mayalso be arranged on the acoustic lining at the rear of the jet engine 52external fan cowl to act as ‘virtual chevrons’. The boundary layerenergisers 20 increase the mixing in the shear layer between the fanstream and the ambient air. This modifies the downstream shock field andturbulent structures within the jet plume with potential benefits forlow frequency cabin noise.

With reference to FIG. 10, the boundary layer energisers 20 may be usedon the core nozzle assembly of a bypass jet engine to promote increasedmixing between fan streams 70 and core streams 72. This willbeneficially modify the shear layer 74 to reduce far-field communitynoise. Furthermore, with reference to FIG. 11 the boundary layerenergiser 20 need not be permanently active and may be selectivelyactivated by virtue of a valve 80. Selectively activating the boundarylayer energisers reduces aerodynamic performance losses associated withtheir operation and offer a significant benefit over current intrusivefixed structure nozzle treatments. The boundary layer energiser may alsobe supplied directly with fan stream air or ram inlet, as by nature theboundary layer energisers have a low intake pressure requirement. Thiseliminates the need for a separate flow source, for example a compressorbleed, and minimises the associated high performance penalties.

With reference to FIGS. 12 and 13, the boundary layer energisers 20 ofthe present invention may be used for a pusher prop-fan configuration 90where the engine 92 is supported by a pylon 94 ahead of the rotor 96.The pylon wake 98 will cause a low energy perturbation to interact withthe rotor. This pressure non-uniformity generates increased noise andblade stress. The nature and region of this interaction is shown in FIG.13( a). The boundary layer energiser 20 can be used in two ways tominimise the pylon wake disturbance that the fan rotor will experience.As shown in FIG. 13( b), the introduction of boundary layer energisersat the pylon leading edge 100 promotes mixing of higher energyfree-stream flow into the surface boundary layer which promotes rapiddecay in the pylon generated wake. In addition the application ofboundary layer energisers at the trailing edge 102 can be used to reducethe pylon chord by the application of a bluffer pylon body, allowingincreased spacing to the fan rotor.

With reference to FIG. 14, boundary layer energisers may be applied tothe leading edge 110 and trailing edge 112 of an aerofoil body 114 inorder to increase lift at incidence. The leading edge 110 boundary layerenergisers would act to delay flow separation, while the trailing edge112 boundary layer energisers would generate a ‘blown flap’ effect,effectively increasing the aerofoil chord and camber. As shown in FIG.15, such a system could be fed via either surface flush inlets 120 or,in the case of the trailing edge boundary layer energisers, via slottedinlets 122 on the lower leading edge positioned to coincide with thebody stagnation point. The boundary layer energisers may be selectivelyactivated by virtue of valves 124, 126 (as shown in FIG. 15( a)) or maybe permanently activated (as shown in FIG. 15( b)). This system isadvantageous over current aerodynamic devices as it would not requirehigh pressure bleed air.

1. A boundary layer energiser for energising a boundary layer flow overa surface, the boundary layer energiser comprising one or more passagesterminating in one or more respective holes provided on the surface,wherein the one or more passages comprises at least one fin configuredso as to generate a vortex flow in the one or more passages such that,when in use, a fluid emanating from the one or more passages flows in avortex.
 2. A boundary layer energiser as claimed in claim 1, wherein theboundary layer energiser comprises a plurality of passages, wherein acentre of each passage is arranged on the surface at the vertex of apolygon.
 3. A boundary layer energiser as claimed in claim 2, whereinthe polygon is a hexagon.
 4. A boundary layer energiser as claimed inclaim 1, wherein the boundary layer energiser comprises a plurality ofpassages, wherein a centre of each passage is arranged on the surface ina linear arrangement.
 5. A boundary layer energiser as claimed in claim1, wherein the passages are arranged such that neighbouring passagesgenerate a vortex in opposite directions to one another.
 6. A boundarylayer energiser as claimed in claim 1, wherein the fin comprises aninwardly facing fin arranged in a spiral, the fin being configured so asto generate a vortex flow in the one or more passages.
 7. A boundarylayer energiser as claimed in claim 1, wherein the flow is provided by aflow source.
 8. A boundary layer energiser as claimed in claim 1,wherein the flow is provided from an opening in a second surface.
 9. Aturbomachine comprising a boundary layer energiser as claimed inclaim
 1. 10. A gas turbine comprising a boundary layer energiser asclaimed in claim
 1. 11. A method of energising a boundary layer flowingover a surface, the method comprising: providing one or more passagesterminating in one or more respective holes provided on the surface;allowing a fluid to flow through the plurality of passages; and forminga vortex with the fluid by virtue of the one or more passages whichcomprise a fin configured such that the fluid emanating from the one ormore passages flows in a vortex.